Wire bonding involves connecting the pads of a semiconductor die to the terminals of a package with a fine (on the order of 0.00125 inch) wire. One common wire bonding technique is referred to as "aluminum wedge bonding". A suitable apparatus for performing such bonding is bonder part number SWB-FA-US-30, available from Shinkawa Corporation of Japan.
A die mounted in an open package is supported on a pedestal, and an automated wire bonder performs the necessary connections between die "pads" and package "terminals", according to pre-programmed instructions. The pedestal is mounted on a rotary table, which rotates the die/package to any of a number of angular positions (azimuths). Hence, the die/package must be securely mounted to the pedestal during the wire-bonding process, and its position relative to the bond tool must be relatively un-ambiguous. A mechanism for raising the die/package off of a carrier to a predetermined height for bonding ("bonding level") may also be included in the wire bonding equipment. The bonding equipment may also include optical equipment for monitoring and controlling the bonding process, such as by locating "target" patterns disposed on the die and package.
Usually the die is square, and is provided with bond pads about its entire outer periphery. Likewise, the package is usually square, and is provided with a corresponding number of terminals about the entire inner periphery of a central opening. A bonding tool, typically including a wedge, places one end of a bond wire in contact with the pad (e.g.) to be bonded, and ultrasonic energy effects bonding of the wire to the pad. The bonding tool then lifts, moves, then lowers itself to another location, and bonds another end of the wire to a package terminal (e.g.). In order to position the bonding tool at various locations in the plane of the die/package, either the rotary table is indexed about its axis, or the bonding tool itself is able to move in an X-Y direction, or both. Evidently, to cover the entire range of pad/terminal connections, it is necessary to rotationally reposition the die/package between bonds.
During the bonding operation, it is necessary to retain the die/package securely upon the pedestal. This is usually accomplished with mechanical clamps, which hold the package to the pedestal. Although mechanical clamping arrangements hold the package securely in place, the have some drawbacks. First, they take up space--usually extending beyond the outer periphery of the package--often interfering with rotational positioning of the die/package (i.e., the clamps bump into other parts of the wire bonder). Second, the clamps themselves have non-trivial mass, which increases the rotational inertia and momentum of the pedestal/die/package. This complicates rotational positioning of the die/package, which may occur at a rate of on the order of one degree per 20 milliseconds.
A common type of semiconductor device assembly has a plurality of pins exiting the bottom surface of the package body, and is termed a Pin Grid Array (PGA). The present invention is especially applicable to PGAs.
The complexity of modern semiconductor devices results in a high number of pins. Pin counts in excess of one hundred are not uncommon. For high pin count packages, the pins are necessarily very thin and closely spaced. Spacing on the order of 0.070-0.100 inch (center-to-center) is not uncommon. Evidently, these pins are delicate, and caution must be exercised in handling the package to avoid damaging the pins so that they can be properly aligned with corresponding holes in a printed wiring board (e.g.) or in a socket to which the packaged device is ultimately mounted. Common damage modes include: 1) physical distortion of the leads, or 2) removal of plating from the leads due to scraping (the leads are commonly gold-plated). Damage to the package body itself, typically a layered ceramic structure, is also a concern.
For PGAs having a plurality of pins extending from the underside of the package, mechanical clamping devices can cause pin damage. Pins are commonly plated (e.g., gold) over base metal (e.g., copper). Pin damage modes include bending of pins, which makes subsequent insertion into a socket of printed wiring board difficult, and scraping the plating, which may mean a "reject" part (e.g., in military specification parts).
One approach to securing the package to the pedestal of a wire bonder, for bonding PGAs, is mounting a cam-actuated test socket to the pedestal, inserting the package, and clamping the pins. This mechanical arrangement, which depends on clamping the pins, can also lead to plating defects and consequent rejection of parts.
Regarding PGAs, for purposes of this discussion there are basically two types: fully-populated and partially-populated. A "fully-populated" pin grid array has pins extending in a number of columns and rows (an array) from substantially the entire undersurface of the package. A "partially-populated" pin grid array also has an array of pins, but has an area clear of pins in a central region of the underside of the package. Evidently, a fully populated PGA can have more pins than a partially populated PGA, for a given size package--all other factors being equal.
In the process of packaging a semiconductor device in a PGA package, it is typical that the semiconductor die is inserted into an opening in the top surface of the package. The package already has pins exiting the bottom surface of the package body, and lead fingers (or "terminals") within the opening in the package body. The lead fingers (terminals) are internally connected to the pins. After the die pads are connected to the lead fingers (such as by wire or tape-automated bonding), a lid is applied over the package opening to seal the device within the package. During these packaging steps, there are many opportunities for causing damage to the pins and/or package while handling and processing the package.
FIG. 1 shows a typical PGA-type semiconductor device assembly 100, for which the present invention is especially pertinent. The assembly includes a square, flat ceramic package body 102 having a top surface 102a and a bottom surface 102b. An opening 104 extends into the top surface of the package. A plurality of pins 106 extend outward from the bottom surface 102b, and are connected (not shown) to terminals (lead fingers, in the case of wire bonding, or bond sites, in the case of TAB bonding) 108 which extend to within the opening. This type of package is referred to as a Pin Grid Array (PGA).
A semiconductor die 114 is inserted into the opening 104, is physically attached (usually adhered) by its back surface 114b to a die attach pad (not shown), and is connected to the exposed ends of the lead fingers 108 by any suitable technique (e.g., wire bonding or tape automated bonding). The die 114 has a front surface 114a containing circuitry (not shown) and conductive pads 120 for input/output connection to the circuitry.
In many cases, selected pins 106, for example one pin at each of the four corners of the package 102, are provided with expanded "stops" 106a spaced from the bottom 102b of the package body 102, so that the package body 102 will sit a prescribed distance above a printed wiring board (not shown) to which the assembly 100 is mounted. For purposes of this discussion, pins 106 having stops 106a are termed "guide pins".
The top surface 102a of the package is has a metallic ring 110 formed about the periphery of the opening 104. After the semiconductor device 114 is electrically connected to the lead fingers (terminals) 108, a lid 116 is secured over the opening 104, "capping" the package. The lid is essentially a flat metal plate, and is evidently slightly larger than the opening 104. The lid is commonly sized to fit over the ring 110. A solder "preform" foil 118, of similar size and shape as the ring 110 is provided between the lid 116 and the ring 110, so that the lid may be secured to the package body 102 simply by heating the entire assembly, causing the preform 118 to seal and secure the lid 116 to the top surface of the package 102, over the opening 104.
Typically, a central area (not shown) on the bottom surface 102b of the package 102 is free of pins, in an area corresponding roughly to the opening 104. This arrangement is known as a "partially-populated" PGA.
FIG. 2 is a view of the PGA 100 of FIG. 1, after wire bonding (prior to assembly of the lid 116 and preform 118). As shown, fine bond wires 122 connect the pads 120 of the die 114 to the terminals 108 of the package 102.
FIG. 3 is a cross-sectional view of a partially-populated PGA, vacuum-chucked to a pedestal. For purposes of this illustration, the opening 104, die 114, bond wires 122 and preform 118 are omitted, for illustrative clarity. Further, this is a "near-sighted" cross section, in that an area 124 clear (void) of pins on the underside 102b of the package 102 is shown without any pins, and the pins 106 behind the clear area 124 are not illustrated. As shown, the package 102 rests atop a pedestal 200. The pedestal 200 has a top surface 202 upon which the clear area 124 of the package rests. The pedestal has an air passage 204 extending from the top surface 202, through the pedestal 200, to a suitable vacuum source 206. In this manner, the package is held reasonably firmly on the pedestal 200. However, there is evidently no easy way to precisely locate the package on the pedestal 200.
Precisely locating the package body on the pedestal is very important in the manufacturing process. It is important that the wire bonder be able to "find" the die pads and package terminals.
It has also been known to vacuum chuck non-pinned packages, in other words packages having leads extending generally in-plane from their side edges, rather than pins extending down from their bottom surface.
While target areas may be imprinted on the die and package, allowing an operator to precisely locate the package and die, such searching takes productive time away from actual bonding. Evidently, resolvers and encoders in the rotary table, or the like, supporting the pedestal, allow the bonder to locate the pedestal with great precision. It is somewhat antithetical that a vacuum chuck, such as is illustrated in FIG. 3, lacks the same precision. It would be preferable not only that a die/package is reasonably precisely located by pedestal, however it may be supported thereby, but also that the location of a die/package mounted to a pedestal is repeatable from package to package in a production operation.
The vacuum-pedestal arrangement shown in FIG. 3 is entirely not suitable for fully-populated PGAs, which do not have a clear area (see 124, FIG. 3) for resting upon the vacuum pedestal. Nor does the vacuum pedestal of FIG. 3 hold the package extremely securely, in that it acts upon only a small central area (124) of the package. Relying entirely on suction and friction, it is possible that the package and die will shift under the torque of rotation, using the vacuum pedestal of FIG. 3. Also, as mentioned above, a vacuum chuck holding the bare undersurface of a package does not provide for good alignment of the package in the bonder.